Package io.vavr

Interface Function0<R>

  • Type Parameters:
    R - return type of the function
    All Superinterfaces:
    java.io.Serializable, java.util.function.Supplier<R>
    Functional Interface:
    This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
    @FunctionalInterface
public interface Function0<R>
extends java.io.Serializable, java.util.function.Supplier<R>
    Represents a function with no arguments.

    • Method Summary

      Modifier and Type Method Description
      default <V> Function0<V> andThen​(java.util.function.Function<? super R,​? extends V> after)
      Returns a composed function that first applies this Function0 to the given argument and then applies Function after to the result.
      R apply()
      Applies this function to no arguments and returns the result.
      default int arity()
      Returns the number of function arguments.
      static <R> Function0<R> constant​(R value)
      Returns a function that always returns the constant value that you give in parameter.
      default Function0<R> curried()
      Returns a curried version of this function.
      default R get()
      Implementation of Supplier.get(), just calls apply().
      default boolean isMemoized()
      Checks if this function is memoizing (= caching) computed values.
      static <R> Function0<Option<R>> lift​(java.util.function.Supplier<? extends R> partialFunction)
      Lifts the given partialFunction into a total function that returns an Option result.
      static <R> Function0<Try<R>> liftTry​(java.util.function.Supplier<? extends R> partialFunction)
      Lifts the given partialFunction into a total function that returns an Try result.
      default Function0<R> memoized()
      Returns a memoizing version of this function, which computes the return value for given arguments only one time.
      static <R> Function0<R> narrow​(Function0<? extends R> f)
      Narrows the given Function0<? extends R> to Function0<R>
      static <R> Function0<R> of​(Function0<R> methodReference)
      Creates a Function0 based on method reference lambda expression Examples (w.l.o.g.
      default Function0<R> reversed()
      Returns a reversed version of this function.
      default Function1<Tuple0,​R> tupled()
      Returns a tupled version of this function.

    • Method Detail

      • constant

        static <R> Function0<R> constant​(R value)
        Returns a function that always returns the constant value that you give in parameter.
        Type Parameters:
        R - the result type
        Parameters:
        value - the value to be returned
        Returns:
        a function always returning the given value

      • of

        static <R> Function0<R> of​(Function0<R> methodReference)
        Creates a Function0 based on Examples (w.l.o.g. referring to Function1): 
        // using a lambda expression
 Function1<Integer, Integer> add1 = Function1.of(i -> i + 1);

 // using a method reference (, e.g. Integer method(Integer i) { return i + 1; })
 Function1<Integer, Integer> add2 = Function1.of(this::method);

 // using a lambda reference
 Function1<Integer, Integer> add3 = Function1.of(add1::apply);

        Caution: Reflection loses type information of lambda references. 

        // type of a lambda expression
 Type<?, ?> type1 = add1.getType(); // (Integer) -> Integer

 // type of a method reference
 Type<?, ?> type2 = add2.getType(); // (Integer) -> Integer

 // type of a lambda reference
 Type<?, ?> type3 = add3.getType(); // (Object) -> Object
        Type Parameters:
        R - return type
        Parameters:
        methodReference - (typically) a method reference, e.g. Type::method
        Returns:
        a Function0

      • lift

        static <R> Function0<Option<R>> lift​(java.util.function.Supplier<? extends R> partialFunction)
        Lifts the given partialFunction into a total function that returns an Option result.
        Type Parameters:
        R - return type
        Parameters:
        partialFunction - a function that is not defined for all values of the domain (e.g. by throwing)
        Returns:
        a function that applies arguments to the given partialFunction and returns Some(result) if the function is defined for the given arguments, and None otherwise.

      • liftTry

        static <R> Function0<Try<R>> liftTry​(java.util.function.Supplier<? extends R> partialFunction)
        Lifts the given partialFunction into a total function that returns an Try result.
        Type Parameters:
        R - return type
        Parameters:
        partialFunction - a function that is not defined for all values of the domain (e.g. by throwing)
        Returns:
        a function that applies arguments to the given partialFunction and returns Success(result) if the function is defined for the given arguments, and Failure(throwable) otherwise.

      • narrow

        static <R> Function0<R> narrow​(Function0<? extends R> f)
        Narrows the given Function0<? extends R> to Function0<R>
        Type Parameters:
        R - return type
        Parameters:
        f - A Function0
        Returns:
        the given f instance as narrowed type Function0<R>

      • apply

        R apply()
        Applies this function to no arguments and returns the result.
        Returns:
        the result of function application

      • get

        default R get()
        Implementation of Supplier.get(), just calls apply().
        Specified by:
        get in interface java.util.function.Supplier<R>
        Returns:
        the result of apply()

      • arity

        default int arity()
        Returns the number of function arguments.
        Returns:
        an int value >= 0
        See Also:
        Arity

      • curried

        default Function0<R> curried()
        Returns a curried version of this function.
        Returns:
        a curried function equivalent to this.

      • tupled

        default Function1<Tuple0,​R> tupled()
        Returns a tupled version of this function.
        Returns:
        a tupled function equivalent to this.

      • reversed

        default Function0<R> reversed()
        Returns a reversed version of this function. This may be useful in a recursive context.
        Returns:
        a reversed function equivalent to this.

      • memoized

        default Function0<R> memoized()
        Returns a memoizing version of this function, which computes the return value for given arguments only one time. On subsequent calls given the same arguments the memoized value is returned.

        Please note that memoizing functions do not permit null as single argument or return value.

        Returns:
        a memoizing function equivalent to this.

      • isMemoized

        default boolean isMemoized()
        Checks if this function is memoizing (= caching) computed values.
        Returns:
        true, if this function is memoizing, false otherwise

      • andThen

        default <V> Function0<V> andThen​(java.util.function.Function<? super R,​? extends V> after)
        Returns a composed function that first applies this Function0 to the given argument and then applies Function after to the result.
        Type Parameters:
        V - return type of after
        Parameters:
        after - the function applied after this
        Returns:
        a function composed of this and after
        Throws:
        java.lang.NullPointerException - if after is null